1. Field of the Invention
The present invention relates to a device for reading a color image utilizing a solid-state image sensor device or the like, and in particular to a color image reading device in which the light from an object is guided to a sensor such as a solid-state image sensor through an imaging optical system and color separation means.
2. Related Background Art
Conventionally a device as shown in FIG. 1 is already known for line scanning an object such as an original document and reading the color image thereof with a solid-state image sensor array, such as a CCD sensor.
The information on a part of an original surface 1 is illuminated by the light from an illuminating light source (not shown), and is read by the reflected light which is guided through an imaging optical system, then is separated into three colors through a three-piece (3P) prism 20 and are focused on three line CCD sensors 21, 22, 23.
However, such conventional structure is inevitably expensive since it requires three independent sensors and the 3P prism requiring high precision in the manufacture thereof. It is also associated with difficulty in manufacture, since the light beams have to be aligned with the sensors 21, 22, 23 independently.
It is therefore conceivable to prepare three line sensors on a same substrate with a certain distance therebetween, thus obtaining a monolithic 3-line sensor 24 shown in FIG. 2. In such a monolithic sensor, the distances s.sub.1, s.sub.2 between three lines 25, 26 and 27 are on the order of 0.1 to 0.2 mm depending on various manufacturing conditions, and the dimensions w.sub.1 .times.w.sub.2 of each single photosensor 28 are on the order of 7.times.7 .mu.m to 10.times.10 .mu.m. FIG. 3 shows a known structure of a color image reading device, utilizing such a monolithic 3-line sensor as the image sensor, wherein, in scanning the information on an original surface 1 in the sub-scanning direction, the light from said surface 1 is guided through an imaging optical system 29, then separated into three light beams of three colors by color-separating beam splitters 30, 31 with a dichroic selective transmission film, and said three light beams are focused on respectively corresponding line sensors of the monolithic 3-line sensor 32.
However, as shown in FIG. 3, a plate thickness t of the beam splitters 30, 31 provides a line distance 2.sqroot.2t on the sensor 32. If said distance is selected on the order of 0.1 to 0.2 mm as explained above, the plate thickness t is about 35 to 70 .mu.m, which is difficult to achieve in manufacture, in consideration of the surface flatness required.
There is also known a color image reading device employing a blazed diffraction grating instead of the dichroic mirror, as disclosed in the U.S. Pat. No. 4,277,138 corresponding to DE 2645075. Said device employs an optical system involving a blazed diffraction grating for color separation.
However such structure only considers the light coming from a point in the object field, but does not pay any attention to so-called image angle characteristics resulting from the presence of a certain image width in the main scanning direction in the object field.